The period from 1945 to 1951 solidified a cellular immunology paradigm for drug allergy, showing that delayed-type hypersensitivity could be transferred between animals via exudate cells and is shaped by exposure history. It also highlights early strategies such as oral tolerance and desensitization, which linked mechanistic insights to clinical management of penicillin and sulfonamide reactions and spurred initial directions in diagnostic testing. Collectively, these studies begin to integrate pathology, immunology, and therapeutics into a coherent framework for safer pharmacotherapy.